330 PLANT GROWTH SUBSTANCES 



would involve or be regulated by only one or merely a few auxins. 



With your forebearance I shall refer to the naturally occurring form- 

 and growth-controlling substances as hormones and to synthetic chemi- 

 cals used for this purpose as growth-regulating substances. This despite 

 the fact that, considering the variability in molecular structure, the 

 synthetics per se do not always, and probably rarely, regulate growth 

 and development but most likely only activate or catalyze in some 

 manner a particular native hormone in plants (21,77). 



It is my intent to discuss briefly certain salient aspects of sexual 

 reproduction with special emphasis on the possible involvement of the 

 growth regulators. Plants with which we have largely worked and those 

 phases of reproduction of greatest present concern to us perforce will 

 be stressed. 



Flower Initiation 



Evidence is being accumulated that the initiation of floral primordia 

 is activated by a special flower-forming hormone. It seems to be produced 

 in the leaves, as in photoperiodism, whence it moves to the apical 

 meristems. The formation of this hormone directly in the meristematic 

 regions is not excluded, as with some temperature effects. While no one 

 has yet succeeded in isolating this hormone it has been tentatively 

 named florigen (13). Comparatively recently the idea has been advanced 

 by Gregory (28) that only a florigenic precursor is formed in the leaves 

 under particular environmental conditions and translocated to the meri- 

 stems where the flower hormone is finally synthesized. Even an anti- 

 florigenic substance, possibly found in leaves on vegetative shoots, has 

 been suggested (48). 



After considering most of the experimental evidence indicating the 

 probability of a flower-forming hormone, up to 1938, Cholodny (14) 

 concluded that not a special but the already known native plant growth 

 substance (heteroauxin), under certain conditions of distribution or even 

 absence, most likely causes the transition of terminal meristems from the 

 vegetative to the flowering condition. Though the situation probably 

 is not as simple and direct as this (37,38), considerable evidence has 

 been accumulated recently that certain synthetic growth regulators can 

 initiate or inhibit flower production, depending on the concentration 

 used. 



Clark and Kerns as early as 1942 (15), Cooper (16), and subsequently 



